Measurements of refractive index sensitivity using long-period grating refractometer Joo Hin Chong, Ping Shum, H. Haryono, A. Yohana, M.K. Rao, Chao Lu, Yinian Zhu * School of Electrical and Electronic Engineering, Network Technology Research Centre, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore Received 27 June 2003; received in revised form 10 October 2003; accepted 23 October 2003 Abstract We report the development and demonstration of a long-period grating refractometer. The principle of operation is based on the using of a long-period grating that is structurally induced by a CO 2 laser, and where the resonance wavelengths are shifted as the refractive index of medium surrounding the cladding of the long-period grating. The different concentrations for three types of solutions (ethylene glycol, salt, and sugar) were experimentally measured, and results show that, as a refractometer, this fiber-based device not only can differentiate chemicals based on their re- fractive index, but it can also become a concentration indicator of a particular chemical solution, and applied in the oil and petroleum industry. Ó 2003 Elsevier B.V. All rights reserved. PACS: 70 Keywords: Concentrations in solutions; Long-period grating; Refractive index; Refractometer; Sensitivity; Fiber Bragg grating 1. Introduction Although the fiber Bragg gratings present an attractive sensing platform, they are limited by either sensitivity to strain or temperature, and of- ten require complex and expensive interferometric technique to detect induced wavelength shifts [1]. Additionally, to use fiber Bragg grating as a re- fractive index sensor (refractometer), the user needs to etch the cladding to gain access to the evanescent field of the guided mode [2]. Long-period gratings (LPGs) could overcome these limitations because they possess high sensi- tivities, simple demodulation schemes, and can be easily configured for multi-parameter measure- ments [3]. For a given LPG, the grating period determines the cladding modes to which light can be coupled. The variation in the grating period and the modal effective indices due to temperature and Optics Communications 229 (2004) 65–69 www.elsevier.com/locate/optcom * Corresponding author. Tel.: +656790-5363; fax: +656792- 6894. E-mail address: eynzhu@ntu.edu.sg (Y. Zhu). 0030-4018/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2003.10.044